1. Field of the Invention
This invention relates to a spatial light modulator applied to a collinear volume holographic storage system, and, especially, to a spatial light modulator that can enhance the convergence of the point spread function of this system.
2. Description of the Prior Art
The collinear holographic storage system is the main development in these years, since the collinear holographic technique has the features of high stability, high reliability and high miniaturability.
In the writing process, the collinear holographic storage system uses a spatial light modulator to generate a signal light and a reference light as a laser light passes the spatial light modulator. After the signal light and the reference light pass a phase modulating mask, the interference will be focused by a lens and recorded homogeneously on a recording medium. In the reading process, the laser light is modulated to a reference light and irradiates on the recording medium to restore the signal via the recorded interference. Finally, the restored light irradiates on a photo-detector and can be read.
FIG. 1a is employed to illustrate the light path of the system in the writing process. The spatial light modulator 200 modulates the incident light 100 to a signal light 110 and a reference light 120, and a lens 300 focus the interference of the signal light 110 and the reference light 120 on a recording medium 400 to be recorded.
FIG. 1b is employed to illustrate the light path of the system in the reading process. The spatial light modulator 200 modulates the incident light 100 to a reference light 120 only, and, after the reference light 120 passes the lens 300, the reference light 120 irradiates on the recording medium 400 to restore the signal light 110 and the reference light 120 via the recorded interference. The restored interference is focused on and irradiates on a photo-detector 600 and can be read.
The structure of a reflective holographic storage system is different from the transmissive system, abovementioned system, but in the similar theory. The rear face of the recording medium 400 is coated with a reflective material 410, and a splitter 700 is set in the light path. In the writing process, the signal light 110 and the reference light 120 pass the splitter 700 and lens 300 to irradiate on the recording medium 400. In the reading process, the reference light 120 is reflected by the reflective material 410 after passing the splitter 700 and lens 300 to restore the interference. The restored interference passes the splitter 700 and will be reflected to a photo-detector 600 for reading. The light paths in the writing process and reading process are shown in FIG. 2a and FIG. 2b. 
The above mentioned spatial light modulator has a disadvantage of poor convergence of the point spread function.